New Probes to Sample Ultra-Low Velocity Zone Structure at the Core-Mantle Boundary Pending NERC Standard Grant 2008 to 2011 £411.868 Sebastian Rost (PI) John Brodholt (co-I) |
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Whole Earth Engine Pending NERC Consortium Grant 2008 to 2011 £877.893 David Gubbins (PI) Sebastian Rost (co-I) Jon Mound (co-I) in collaboration with Univ. Durham, Univ. Exeter, UCL, and Univ. Leeds (Maths) |
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| Continental and oceanic upper mantle structure from seismic array data NERC New Investigator 2008 to 2011 £ 86.453 Sebastian Rost (PI) |
Velocity profiles of the
Earth's upper mantle are characterized by discontinuous jumps of the
seismic velocities. The main velocity discontinuities (or simply
discontinuities) are located at depths of approximately 410 and 660 km.
Both of these discontinuities can be explained by solid-solid phase
transitions in the major olivine component of the mantle material.
Nonetheless, the minor constituents of the mantle material will
introduce additional, mostly smaller, discontinuous jumps of the
velocities at different depths. These transitions complicate the
seismic image of the upper mantle structure. High resolution studies
are necessary to detect these discontinuities and to image the fine
scale structure of the upper mantle with strong implications for the
mineral-physical constitution of the Earth's mantle and geodynamical
modelling of dynamics and evolution of Earth's mantle.
We will use traveltime and waveform information from data recorded at seismic arrays located in India and Australia to resolve the structure of the upper mantle beneath northern Australia and northern and eastern India. Major earthquake belts are located in a distant range from these arrays that allows the study of the seismic wave triplications due to the velocity increases at the discontinuities. Several thousand earthquakes recorded at the arrays will be collected to achieve a dense coverage of the study area. |
Investigating the Relationsship between Plume Dynamics and ULVZ Geometry NSF Collaborative Studies of the Earth Deep Interior 2005 to 2007 $178,946 Allen McNamara (PI) Sebastian Rost (co-PI) Edward Garnero (co-PI) In collaboration with Michael Manga - University of California Berkeley |
The ultra-low velocity zone (ULVZ) is a thin ( ~10 km) layer in some regions of the lower-most mantle immediately above the boundary with the outer core that is characterized by a dramatic reduction in seismic wave speeds. The cause of this reduction in wave speed is uncertain, but likely possibilities include a small degree of mantle melting and chemical heterogeneity created by reactions between silicate minerals in the mantle with iron in the core. Previous, lower resolution studies that characterized the geographic extent of the ULVZ have hinted at large regional patches, however, recent, higher resolution observations made by members of this group have revealed a potentially smaller-scale structure than originally thought. A small, isolated pocket of more-dense ULVZ material was discovered in a region that was previously thought to contain a much larger, continuous ULVZ layer. Work proposed here involves a collaboration of seismologists and both numerical and laboratory geodynamicists at Arizona State University and the University of California Berkeley. The ultimate goal of this work is to determine how upwelling mantle plumes originating from the core-mantle boundary affect the local geometry of the ULVZ, and one exciting possibility to examine is whether seismically detectable pockets of ULVZ can be used as markers to determine the source region for nearly seismically-invisible mantle plumes. On the seismology front, the ULVZ will be studied at much higher resolution than before with the goal of determining whether previously-thought-continuous ULVZ regions are instead composed of isolated pockets of more-dense regions. Geodynamically, numerical and laboratory experiments will be used to determine whether observed isolated pockets of ULVZ material are related to mantle plumes, and if so, what is the relationship between the morphology of ULVZ material and the flow patterns associated with mantle plume source regions? Finally, proposed work will focus on determining whether the dynamically-predicted and observational constraints can be used to differentiate between competing hypotheses of partial melting and chemical heterogeneity as a cause of the ULVZ. |
Fine Scale Structure of the Upper Mantle Using Array Technologies NSF Geophysics 2005 to 2007 $132,713 Sebastian Rost (PI) Edward Garnero (co-PI) |
The internal boundaries (seismic discontinuities) of Earth's upper mantle down to depths of the mantle transition zone at 700 km contain important information about the thermal, compositional and dynamical state of the Earth's interior. High resolution studies of this region of the Earth provide important constraints on the internal works of this planet. Most upper mantle and transition zone studies image structure at intermediate to long wavelength (500 - 1000 km), thus lacking resolution to study structure at shorter scales (10's km or less). This proposal focuses on using seismic array data and array processing technologies to improve fine scale resolution of this region. A seismic array is a seismometer network that permits analysis of Earth structure though time series stacking to enhance signal to noise ratios of coherent arrivals over incoherent energy. Improved signal to noise ratios allows studying Earth structure at short scale lengths. Arrays permit measurement of the incidence angle of the wavefield, therefore allowing better phase characterization and identification. Using data from two arrays located in Australia (Warramunga Array) and India (Gauribidanur Array) we target three main objectives: 1) Resolving fine scale structure of the 410-km and 660-km discontinuities through array analysis of the Pwave triplication beneath Australia and India; 2) Studying the nature of the 210-km discontinuity beneath oceanic regions using PP scattering; 3) Detection, characterization and modeling of reflectors in the uppermost lower mantle through Pto- P and S-to-P reflections and conversions. Using a large dataset including hundreds of earthquakes recorded at the two arrays will give us an unprecedented resolution beneath northern Australia and beneath north and northeast India. The higher resolution will facilitate a more direct comparison to mineral physical and geodynamical implications including the distribution of water in the mantle, key features of the mantle transitions zone discontinuities (thickness, seismic velocity gradient, and discontinuity sharpness), the presence or absence of an intermediate depth discontinuity at 520-km and the existence of partially molten material above 410-km depth. |
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